How to avoid blurry post pictures – thumnail problem

Dear Site Origin Team,

Order number:

http://www.fairandgreen.de/weinwelt/

after different trials I am still left out without a propriate solution.

By posting a post image the image becomes blurry. The image has been size 1050×500 px before uploading. WordPress sizes it down to 720 x 540 – by posting it as a post picture it is beeing streched to 1048×786. How can I avoid this streching?

– I already tried to change the settings – media sizes.

– I already tried to add the size in the theme settings – function

set_post_thumbnail_size(‘720, 380, true’);
add_image_size(‘vantage-thumbnail-no-sidebar’, 1080, 380, true);
add_image_size(‘vantage-slide’, 960, 480, true);
add_image_size(‘vantage-carousel’, 272, 182, true);
add_image_size(‘vantage-grid-loop’, 436, 272, true);

Was this the right way? – Or what can I do because pictures are still blurry. They stay blurry even if I do a new upload and streching stays.

Maybe it is more easy… but I can’t find it out – would be glad to find help

We are using Vantage Premium.

Thank you very much for your support

Ines Derra